1. Field of the Invention
The present invention relates to an interferometer, a demodulator including this interferometer, and to a splitting element used in this interferometer.
Priority is claimed on Japanese Patent Application No. 2007-080447, filed on Mar. 27, 2007, the content of which is incorporated herein by reference.
2. Description of the Related Art
Generally in an interferometer, an incident beam of light is split into a plurality of split beams, and after passing through different optical paths the split beams are caused to interfere, and the interference fringes or similar are measured. One type of interferometer is a delayed interferometer, in which, relative to one split beam, another split beam is delayed, and interference is caused. FIG. 11 shows the configuration of a delayed interferometer of the conventional art. The delayed interferometer 100 shown in FIG. 11 is a Michelson-type delayed interferometer, including a beam splitter 101 and planar mirrors 102 and 103
The beam splitter 101 is a plate shape member formed by for example forming a prescribed multilayer dielectric film 101b on glass substrate 101a. This beam splitter 101 both reflects and transmits incident light L100, to split the light into split beams L101 and L102 having a prescribed intensity ratio (for example, 1:1). The beam splitter 101 combines the split beams L101 and L102 after being reflected by the planar mirrors 102 and 103 respectively to cause interference, and also splits the interference light obtained by interference at a prescribed intensity ratio (for example, 1:1). This beam splitter 101 is positioned such that the incident light L100 is incident at a prescribed angle (for example, 45°) on the surface on which the multilayer dielectric film 101b is formed.
The planar mirror 102 is positioned in the optical path of one of the split beams L101 split by the beam splitter 101, such that the reflecting face is perpendicular to the optical path. The planar mirror 102 reflects the split beam L101 from the beam splitter 101 toward the beam splitter 101. The planar mirror 103 is positioned in the optical path of the other split beam L102 split by the beam splitter 101, such that the reflecting face is perpendicular to the optical path. The planar mirror 103 reflects the split beam L102 from the beam splitter 101 toward the beam splitter 101. In the delayed interferometer 100 shown in FIG. 11, the planar mirrors 102 and 103 are positioned such that the optical path length of the split beam L101 is longer by a prescribed length than the optical path length of the split beam L102.
In the above configuration, when the incident light L100 is incident on the delayed interferometer 100, the incident light is split by the beam splitter 101 into split beams L101 and L102. The split beams L101 and L102 are reflected by planar mirrors 102 and 103 respectively, and are again incident on the beam splitter 101. The optical path length of the split beam L101 is longer by a prescribed length than the optical path length of the split beam L102, so that the split beam L101 is delayed by a prescribed length of tine relative to the split beam L102. Then, the split beams L101 and L102 are combined at the beam splitter 101 and caused to interfere, and by this means phase comparison is performed between the split beam L102 and the split beam L101 which is delayed by the above time duration. The interference light having an intensity according to the comparison result is output as output beams L103 and L104.
FIG. 12 shows a modified example of the delayed interferometer of the conventional art shown in FIG. 11. The delayed interferometer 200 shown in FIG. 12 is a Michelson-type delayed interferometer including right-angle prisms 202 and 203 instead of the planar mirrors 102 and 103 shown in FIG. 11. By employing right-angle prisms 202 and 203, the outbound and inbound paths of the split beams L101 and L102 can be shifted (given an offset). Further, the position of emission of the output beam L103 can also be changed relative to the position of incidence of the incident light L100.
The above-described delayed interferometers 100 and 200 are provided in demodulators of WDM (Wavelength Division Multiplexing) optical communication systems, in which optical signals modulated by for example differential phase shift keying (DPSK) or another modulation method are subjected to wavelength division multiplexing (WDM) and transmitted. Differential phase shift keying is a modulation method in which a signal is modulated by relative phase difference with the phase of the preceding signal to perform modulation. When using a delayed interferometer 100 and 200 in a demodulator, by means of the following measure, the modulated light can be demodulated. That is, each of the optical path lengths is set such that the split beam L101 is delayed, relative to the split beam L102, by a time duration equivalent to one bit of the modulation rate of the modulated light. The modulated light is made incident as the incident light L100.
Details of a demodulation device including a delayed interferometer of the conventional art in a WDM optical communication system may for example be found in Published Japanese Translation of PCT Application 2004-516743 (PCT Publication No. WO 02/51041).
Ideally, the beam splitter 101 employed in the delayed interferometers 100 and 200 splits the incident light L100 at a prescribed intensity ratio, regardless of the polarization state of the incident light L100 (S-polarized or P-polarized), and without causing a relative phase difference between the split beams. However, actual beam splitters 101 are not ideal. Hence relative phase differences between the split beams L101 and L102 occur, according to the polarization state of the incident light L100, arising from imperfections in the beam splitter 101.
Due to such phase differences, there is the problem of occurrence of phenomena in which the phase of a delayed interferometer 100 or 200 changes depending on the polarization state of the incident light L100 (PDFS: Polarization-Dependent Frequency Shift). Polarization-dependent frequency shifts (PDFS) are not a problem inherent only in delayed interferometers such as the delayed interferometers 100 and 200 shown in FIG. 11 and FIG. 12, but is a problem which occurs in interferometers in general which employ splitting elements such as half-mirrors, beam splitters, and similar.